Method of achieving variable performance of an electric generator

ABSTRACT

Disclosed is a method of operating a large electric generator, the generator having a rotor arranged along a centerline of the generator, a core arranged coaxially and surrounding the rotor; a plurality of stator windings arranged within the core; a stator frame arranged to fixedly support the core and rotationally support the rotor; a gas cooling system that circulates a cooling gas within the generator, the method steps including circulating a cooling liquid that cool the stator windings; sensing an output parameter of the generator; comparing via a control system the sensed output parameter of the generator to a predetermined scheme; and sending a control signal to an adjusting device in accordance with the control system comparison.

CLAIM OF PRIORITY AND CROSS REFERENCE TO RELATED APPLICATIONS

The instant application claims benefit and priority to U.S. provisionalapplication 62/105,798 filed on Jan. 21, 2015 and is incorporated hereinin its entirety. Furthermore, this application is related toapplication's ______; ______ ______ and ______ (attorney docket #'s2015P20069US, 2015P00589US01, 2015P20070US and 2015P20071 US) filedconcurrently.

FIELD OF THE INVENTION

The present invention relates to high voltage power generation equipmentand, more particularly, to a method of achieving variable performance ofan electric generator.

BACKGROUND OF THE INVENTION

Dynamoelectric high-voltage machines and/or high-voltage systems, suchas electrical generators in power plants, represent a substantialcapital investment. Furthermore, it is desirable to avoid over-sizing interms of the performance capability of the generator

Due to the serious technical issues presented to the power generationindustry due to part-load and varying-load operation, there is asignificant need for a simple, low cost generator with a variablemaximum efficiency/variable maximum out-put as well as a generator withreduced winding insulation thermal stresses and greater winding life.

SUMMARY OF THE INVENTION

One aspect is a method of operating a large electric generator, thegenerator having a rotor arranged along a centerline of the generator, acore arranged coaxially and surrounding the rotor; a plurality of statorwindings arranged within the core; a stator frame arranged to fixedlysupport the core and rotationally support the rotor; a gas coolingsystem that circulates a cooling gas within the generator, the methodsteps comprising circulating a cooling liquid that cool the statorwindings; sensing an output parameter of the generator; comparing via acontrol system the sensed output parameter of the generator to apredetermined scheme; and sending a control signal to an adjustingdevice in accordance with the control system comparison.

A further aspect includes further comprising the step of adjusting apressurizing system in accordance with the control signal.

A further aspect includes the adjusting device permits the increase ordecrease of cooling gas pressure via the pressurizing system.

A further aspect includes the adjusting device adjusts circulation ofthe cooling liquid.

A further aspect includes the adjusting device is a control valve.

A further aspect includes the cooling ability of the cooling gas is afunction of the sensed output parameter of the generator.

A further aspect includes the predetermined scheme minimizes temperaturefluctuations of the stator and rotor windings.

A further aspect includes the predetermined scheme minimizes thermalstresses of the stator and rotor windings.

A further aspect includes the predetermined scheme maximizes theefficiency of the generator.

A further aspect includes the predetermined scheme maximizes the poweroutput of the generator.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the present invention, it is believed that thepresent invention will be better understood from the followingdescription in conjunction with the accompanying Drawing Figures, inwhich like reference numerals identify like elements, and wherein:

FIG. 1 depicts a gas cooled generator system, cooling gas pressurizingunit and associated control system;

FIG. 2 is depicts a stator core and a winding coil residing within thecore;

FIG. 3 depicts an insulated coil;

FIG. 4 depicts an end view of coil showing the conductors;

FIG. 5 depicts various coil thermal responses due to a step change ingenerator load; and

FIG. 6 depicts comparison of efficiency vs. load for equivalent ratedgenerators.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiment,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration, and not by way oflimitation of the embodiment. It is to be understood that otherembodiments may be utilized and that changes may be made withoutdeparting from the spirit and scope of this disclosure.

Currently, an electrical generator is configured or sized for therequired maximum power output (MVA) which represents a particularphysical configuration of the generator and an associated operatingefficiency and cost to manufacture. However, modern power plants now seegreater variation in required power output demand from the grid andassociated operating conditions. In particular, one common variationmodern power plants are experiencing is the requirement to operate atpart-load conditions. For example, within the United States, generatorsdo not typically operate at full load, rated power factor conditions.Instead, these generators typically operate approximately 80% of fullload and at a variety of power factors. Part-load conditions can be anyoperating condition less than 100% power output. However, part-loadoperation typically is within the range of 20%-80% of maximum poweroutput. Additionally, part-load conditions may be more specifically60%-80% of maximum power output.

Furthermore, the state of the art of electric generator technologyrepresents three primary configurations for cooling the generator rotorand stator, i) air cooling for both the rotor and stator, ii) hydrogencooling for both the rotor and stator, and iii) water cooling for thestator coils and hydrogen cooling for the rotor and stator core. Aircooled electric generators are lower cost and mechanically less complexthan hydrogen cooling and/or the combination of hydrogen and watercooling.

Advances in power plant prime mover power output capability have led toa demand for generator ratings that extend beyond the current capabilityof low-cost, conventional air-cooled units. Conventional air-cooleddesigns in the industry have generally been limited to approximately 350MVA.

Typically, for applications above 350 MVA, a pressurized hydrogen-cooledgenerator is required. Although hydrogen is an excellent coolant,hydrogen-cooled generators are inherently more complex, more expensiveto build and more expensive to operate than air-cooled generators due tothe need to (i) prevent the mixing of hydrogen and air (i.e. complicatedand expensive sealing systems and operating procedures) and (ii) theneed to contain a potential hydrogen explosion within the generator (avery robust pressure vessel frame capable of containing the explosionpressure).

Furthermore, with the typical gas-cooled (air or hydrogen) generator,the cooling gas circuit is designed for 100% power output of thegenerator. However, when operating at part-load, the generator is notproducing full power but the windage losses from the cooling gas circuitremain constant. Therefore, the generator and the power plant areoperating at sub-optimal efficiency.

In addition to part-load operation, another common variation modernpower plants are experiencing is the requirement to change loadfrequently. An issue associated with varying-load operation of thegenerator is large variations in operating temperatures of the statorand rotor windings due to the variations in power output to match theoperational load demand. The variations in operating temperatures of thewindings results in excessive thermal stress cycles of the windingcomponents and premature aging of the insulation materials due to thechange from one operating state of the generator to another, see FIG. 5.

Heat from traditional gas cooled generators is removed throughconvective heat transfer from the generator active components to thecooling gas. In the most simplistic terms, heat is primarily removedfrom generator components via convective heat transfer which is definedby:

Q=m*Cp*dTg.  Eq. (1), and

Q=h*A*dTs, where h is f(m,k)  Eq. (2).

As can be seen by examination of Eq. (1) above, heat to be removed fromthe generator (Q) can be increased by increasing cooling gas mass flowrate (m), cooling gas specific heat (Cp), and/or the cooling gastemperature rise along the flow-path (dTg). Eq. (2) shows that Q canalso be increased by increasing the heat transfer coefficient (h) aswell as the temperature differential between the component temperatureand the cooling gas (dTs). Furthermore, the heat transfer coefficient isa direct function of m and gas thermal conductivity (k). Heat transfercan also be increased by increasing the surface area (A) of thecomponent to be cooled, but this is typically constrained by otherdesign criteria. Increasing the cooling gas pressure from 0 to 1 bargage effectively doubles m and therefore the cooling capacity of thegas. The dTs is typically limited by the material properties used in theconstruction of the generator components. However, dTs is generallymaximized while meeting design requirements such as low and high cyclefatigue life and acceptable thermal stresses. The Cp and k are dependenton the selected cooling gas chemistry. Typically, Cp and k are greaterfor lighter gases like hydrogen or helium, and less for heavy gases suchas air, molecular nitrogen or carbon dioxide. Historically, generatordesigns have tried to optimize heat removal through the optimalselection of Cp and k of the cooling gas along with the cooling gasdensity by determining a fixed operating gas pressure.

It can be observed that the largest benefit from pressurization occursfrom 0 bar gauge to 1 bar gauge pressure, i.e. a doubling of cooling gaspressure results in a doubling of the heat transfer capacity of thecooling gas. Increased benefit due to higher pressurization diminishesas the cooling gas pressure increases above 1 bar gauge pressure.However, further pressurization to approximately 2 bar gauge pressuremay still provide acceptable heat transfer and performance gains.However, the drawback of increased pressurization is increased fluidpumping losses (known as windage) due to the increased density of thecooling gas.

The positive and negative impacts of cooling gas pressurization must bebalanced to obtain an optimal configuration. Gas pressurizationincreases heat removal capacity, but also increases the heat generationthrough windage/friction and therefore reduces efficiency of thegenerator.

Referring to FIG. 1, an embodiment provides a gas cooled generator 1.The generator 1 includes a rotor 2 arranged along a centerline 3 of thegenerator 1. The generator further includes a stator core 5 arrangedcoaxially with the rotor 2 and at least partially surrounds the rotor 2.

As can be seen in FIGS. 2 and 3, the stator core 5 supports a pluralityof electrical conducting coils 100. The plurality of coils 100 arecommonly known within the power generation industry as windings 6. Thecoil 100 typically consist of electrical conductors 103 surrounded byelectrical insulation 101 to electrically isolate the electricalconductors from the stator core 5 and other surrounding components ofthe generator 1. One of ordinary skill in the art of electricalgenerator design can readily appreciate that there are several commonschemes for cooling the windings 6.

For example, as can be seen in FIGS. 2, 3 and 4, coil 100 can be cooledindirectly by conductive heat transfer to the stator core 5. That is,the winding 6 is not cooled directly by a cooling medium. Rather, theheat generated within the coil 100 due to operation is conducted to thestator core 5 and the stator core 5 is then cooled by a cooling medium.

In a further example, and as can be seen in FIG. 4, another means ofcooling the coil 100 is direct cooling, where cooling passages 102 areformed within or adjacent to the coil 100 itself. The cooling passages102 can be formed integrally with and as an electrical conductor 103 orthe cooling passages 102 can be formed discretely from the electricalconductor 103 as a separate component.

Referring again to FIG. 1, the embodiment further provides a lowpressure pressurizing unit 7 configured to vary an operating pressure ofa non-explosive cooling gas 10 typically between atmospheric pressure (0bar gauge) and 1 to 2 bar above atmospheric (1 to 2 bar gauge) asdetermined by control system 21. The pressurizing unit 7 may incorporatea compressor to pressurize the cooling gas, a pressure reservoir tostore a large volume of compressed gas to facilitate the rapidpressurization of the generator 1 as determined by the control system21, and an adjusting device 8 where the adjusting device 8 may be acontrol valve. Furthermore, the compressor could be a standalone deviceor could be represented an extraction of compressed gas from anotherdevice like a gas turbine compressor or steam turbine.

The control system 21 continuously senses a state of the power output ofthe generator to the electrical grid 22. The control system 21 may sensethe load on the generator 1 from typical control room signals, forexample, such as terminal voltage, phase current demand, and powerfactor. The control system 21 then compares the sensed generator outputsignal 22 to the load demand of the generator 1 and determines anoptimum pressure of the cooling gas 10 based on a predetermined controlscheme. Once the optimum pressure of the cooling gas 10 is determined bythe control system 21 in conjunction with the predetermined scheme, acontrol signal 23 is generated by the control system 21 and transmittedto the pressurizing unit 7. The pressurizing unit 7 then adjusts theoperating pressure of the cooling gas 10 in accordance with the controlsignal 23. The pressure control scheme can be customized to maximizegenerator efficiency.

A further aspect may utilize liquid cooling of the stator coil 100 toprovide additional operational flexibility, improved efficiency andreliability of the coil 100, see FIGS. 4 and 5. The combination ofdirect liquid cooling of the stator coils 100 with low pressurenon-explosive gas cooling of the stator core 5 and the rotor 2 resultsin greater operational flexibility and efficiency of the generator 1.Liquid cooling of the stator coils 100 eliminates the heat load from thestator coils 100 on the cooling gas 10 which thereby reduces the heatdemand on the overall cooling gas system (i.e. cooling unit 9, coolinggas 10, blower 11 and pressurizing unit 7). Therefore, the losses due tothe gas cooling system can be minimized.

Additionally, the control system 21 may further control the pressurizingunit 7 along with the liquid cooling of the stator coils 100 to regulatethe generator component temperatures in accordance with thepredetermined scheme to minimize thermally induced stresses of thevarious components like the stator coils 100 and rotor 2, so as tomaximize the life of the low pressure non-explosive gas generator 1 inenvironments where load cycling is excessive, see FIG. 5.

A further aspect utilizes a pressure boundary member 4 to contain thepressure of the cooling gas 10. The pressure boundary member 4 may beformed as a frame from thin plate material or as a semi-flexiblepressure containment device formed from film or fabric, all of which maybe made from a polymer, metallic or composite materials of sufficientconstruction to withstand the operating environment of the generator,i.e. temperature, pressure, vibration and environmental considerations.Typically, metallic thin plate material utilized for the low pressureframe or pressure boundary member 4 may be between approximately 5 mmand 15 mm thick and film or fabric may be between approximately 1 mm and5 mm thick. Typical hydrogen cooled generators have much thicker steelwalled frames due to the fact that the hydrogen cooling gas pressure istypically 3 to 5 bar gauge pressure, as well as the frame must be strongenough to contain the internal pressure due to an explosion of thehydrogen cooling gas. In order for the typical hydrogen cooled generatorframe to contain the pressure for an internal explosion of hydrogencooling gas, the frame walls are typically approximately 19 mm to 51 mmthick. The present frame or pressure boundary 4 is intended only towithstand the relatively low pressure of 1 to 2 bar gauge pressure anddue to the non-explosive nature of the cooling gas; the frame 4 does nothave to contain explosion pressures. Therefore, the low pressure frameneed 4 only be sized and configured to meet the operating conditions ofa non-explosive cooling gas low pressure generator and not the explosionpressures resulting in a structurally thinner frame or pressure boundarymember 4.

Additionally, through the benefit of low pressure gas cooling to amaximum of 1 to 2 bar gauge pressure, the entire generator can be sizedmuch smaller than an equivalent output traditional air cooled generator.For example, the present low pressure non-explosive gas generator 1 canbe smaller in physical size than present air cooled generators by 10 to35%. Preferably, the present low pressure non-explosive gas generator 1could be 35% smaller in physical size than a typical air cooledgenerator, or alternatively, the present low pressure non-explosive gasgenerator 1 could achieve a 50% to 60% increase in power output comparedto a typical air cooled generator of the same size or physicaldimensions.

Furthermore, if the power output demands of the generator exceed thepower output limits of the generator 1, the cooling gas pressure can bequickly and easily increased up to 1 bar gauge pressure and furthermoreto a maximum of 2 bar gauge pressure for additional generator poweroutput capacity. For example, by varying gas pressure between 0 and 1bar gauge, an increase of approximately 50% of additional power outputcapability with no other modification required to the generator 1, seeFIG. 6.

Furthermore, the cooling system of the low pressure non-explosive gasgenerator 1 is physically smaller than that of a typical air cooledgenerator due to the combination of pressurized air cooling and awater-cooled stator winding. Therefore, the windage losses for the lowpressure non-explosive gas generator 1 are greatly reduced as comparedto an equivalent power output traditional air cooled generator. At poweroutputs less than 100% rated output, the cooling gas pressure of the lowpressure non-explosive gas generator 1 can be reduced to obtain theoverall efficiencies that improve at partial load, see FIG. 6.

In a further aspect, the low pressure non-explosive gas generator 1 mayutilize alternate gasses as the cooling medium. Such gases are listed inTable 1 below.

TABLE 1 Refrigerant Number Name — Air R-704 Helium R-702 Hydrogen R-720Neon R-728 Nitrogen R-740 Argon R-134a Tetrafluoroethane R-152aDifluoroethane R-717 Ammonia R-744 Carbon Dioxide

Additionally, the gasses identified in Table 1 may also be mixed invarious combinations to provide specific performance characteristics.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

What is claimed is:
 1. A method of operating a large electric generator,the generator having a rotor arranged along a centerline of thegenerator, a core arranged coaxially and surrounding the rotor; aplurality of stator windings arranged within the core; a stator framearranged to fixedly support the core and rotationally support the rotor;a gas cooling system that circulates a cooling gas within the generator,the method steps comprising: circulating a cooling liquid that cool thestator windings; sensing an output parameter of the generator; comparingvia a control system the sensed output parameter of the generator to apredetermined scheme; and sending a control signal to an adjustingdevice in accordance with the control system comparison.
 2. The methodof claim 1, further comprising the step of adjusting a pressurizingsystem in accordance with the control signal.
 3. The method of claim 1,wherein the adjusting device permits the increase or decrease of coolinggas pressure via the pressurizing system.
 4. The method of claim 1,wherein the adjusting device adjusts circulation of the cooling liquid.5. The method of claim 1, wherein the adjusting device is a controlvalve.
 6. The method of claim 1, wherein the cooling ability of thecooling gas is a function of the sensed output parameter of thegenerator.
 7. The method of claim 1, wherein the predetermined schememinimizes temperature fluctuations of the stator and rotor windings. 8.The method of claim 1, wherein the predetermined scheme minimizesthermal stresses of the stator and rotor windings.
 9. The method ofclaim 1, wherein the predetermined scheme maximizes the efficiency ofthe generator.
 10. The method of claim 1, wherein the predeterminedscheme maximizes the power output of the generator.